He is the skeleton of a humongous dragon that resides at the bottom of the Pit of 100 Trials. Before he was there, he was the Shadow Queen's pet. It is unknown what his name was before he became a skeleton. When Mario got to the hundredth floor, he fought him. Bonetail was then defeated, and coughed up a treasure chest conatining the Return Postage Badge. After his death he was not seen again in the Pit of 100 Trials.
Bonetail's stats exceed that of The Shadow Queen: 200 HP, 8 ATK, and 2 DEF, though he doesn't have any separate hands/appendages that attack. This possibly makes the fight easier. His breath has several colors and effects: Green confuses Mario, ice freezes him, purple shrinks him and white puts him to sleep. He can also heal 20 HP, twice. His text box looked identical to that of the Shadow Queen, and he only howled like a wolf.
- Unlike Hooktail and Gloomtail who are capable of speech, Bonetail is only capable of making growls and dog-sounds.